Thin and light liquid crystal display (LCD) devices are being employed in a widening range of applications including displays for mobile information terminals. An LCD device is a passive device which does not emit light itself but changes its light transmittance and can thus be used in displays. The LCD device requires a very low driving voltage of just a few volts. In a LCD device, a reflector is provided underneath the LCD device to display information by taking advantage of the reflection of external light, dispensing with the need to provide power for background lighting. The result is a display device with an extremely low power consumption.
A conventional reflective color LCD device comprises a liquid crystal cell with a color filter, and a pair of polarizer films disposed to sandwich the liquid crystal cell. The color filter is provided on one substrate of the liquid crystal cell, and a transparent electrode is further formed on the color filter. Color information may be displayed by applying voltage to the liquid crystal cell to change the orientation of liquid crystal molecules and controlling the light transmittance of the color filter for each color.
The light transmittance of one polarizer film is only about 45%. Transmittance of polarized light parallel to the absorption axis of the polarizer film is about 0%, and transmittance of polarized light perpendicular to the absorption axis is about 90%. In a reflective LCD device employing two polarizer films, the light passes through the polarizer film four times. Accordingly, the light usage rate of a conventional type of LCD device will be as follows when the light absorbed by the color filter is ignored: EQU (0.9).sup.4.times.50%=32.8%.
This shows that the light usage rate is a maximum of about 33% even for a monochrome LCD panel which does not use the color filter. If the color filter is added to this type of LCD to create a color display, its light usage rate further drops, resulting in an inability to achieve sufficient brightness for practical use.
To achieve a brighter display, several configurations are proposed concerning the provision of only one polarizer film on the upper surface of the liquid crystal cell, and sandwiching the liquid crystal cell between this single polarizer film and a reflector. (For example, refer to Japanese laid-open patent application nos. H7-146469 and H7-84252). In this case, the light passes through the polarizer film only twice. If the light absorption at the color filter is ignored, the light usage rate will be as follows: EQU (0.9).sup.2.times.50%=40.5%
Compared to the configuration of using two sheets of polarizer film, the light usage rate may be improved by a maximum of about 23.5%. However, since only one sheet of polarizer film is used, an achromatic display of black and white is difficult to achieve. In particular, it may be difficult to display black, the low reflective achromatic color.
Other conventional types include a reflective color LCD device using the birefringence of a twisted nematic liquid crystal layer and a polarizer film for color display (Japanese laid-open patent application no. H6-308481) which eliminates the use of a color filter, and a color LCD device using the birefringence of a liquid crystal layer and phase difference film (Japanese laid-open patent application nos. H6-175125 and H6-301006). These types of LCD devices enable the achievement of reflectance at a practical brightness even if two sheets of polarizer film are used, since no color filter is required. However, a birefringent color display is theoretically difficult to apply to multi-level gradation and multicolor displays such as when displaying 16 level gradations of 4096 colors and full color of 64 level gradations. In addition, the color purity and color reproducible range are narrow.